This invention relates to a method for uniformly heating plastically deformable material, which comprises electrically conductive particulate matter, by induction heating. Induction heating is accomplished by placing the plastically deformable material in proximity with an induction device through which an electric current of appropriate frequency is passed, thus causing induction of an electric current or hysteresis loss within the material. Induction of such an electric current or causing of hysteresis loss within the electrically conducting particulate matter of the plastically deformable material generates heat thereby causing uniform heating of the plastically deformable material.
This heat generation causes the plastically deformable material to heat uniformly which leads to stiffening, or rigidification of formed plastically deformable material at least to the point that it is capable of being easily handled without ready deformation resulting from the handling. The observed stiffening probably results from the heating causing either gelation of a thermally gelling component of the material, at least partial drying or solidification, or a combination of the two actions. With continued induced heating it should be possible to cause full curing of binders, burnout of the binders and, finally, sintering of the formed article.
Induction heating is a recognized method for causing surface heating of objects or materials and has been used for metal melting, welding, and bonding. Induction heating has not been recognized as a method for even heating of objects. Such even heating is an important aspect of the practice of this present invention. An example of the use of induction heating is found in U.S. Pat. No. 3,352,951 issued to Sara Nov. 14, 1967. Sara teaches a method for providing high density refractory carbide articles by forming carbide material, without liquid or binder, to a desired shape and encapsulating the article within electrically conductive material (a "receptor"), which must have a higher melting point than the material within the receptor. The encapsulated article is then sintered at a temperature just below the melting point of the material itself. The sintering occurs under an inert atmosphere and is accomplished by inductive heating of the electrically conductive capsule which surrounds the formed carbide article. Induction heating has not, however, been known to be used to create a rigid formed green or unfired body shaped of particulate electrically conductive material mixed with plasticizing ingredients such as organic binder and a liquid, of which the former may be gelled and from which the latter is volatilized to produce stiffening and drying of such mixture.